Process for oxidizing a benzenoid structure carrying a methyl substituent

ABSTRACT

A process for selectively oxidizing the methyl substituent on a benzenoid structure carrying a nuclear methyl substituent and a higher alkyl substituent which involves heating the benzenoid structure with oxygen in the presence of cobaltic ions dissolved in a lower carboxylic acid solvent.

United States Patent Onopchenko et a]. Oct. 7, 1975 PROCESS FOR OXIDIZING A BENZENOID [58] Field of Search 260/524 R STRUCTURE CARRYING A METHYL SUBSTITUENT References Cited [75] Inventors: Anatoli Onopchenko, Monroeville; UNITED STATES PATENTS Johann Schuh, Pittsburgh, 2,302,462 ll/l942 Palmer et a1 260/524 both of P34 Richard Seekircher, FOREIGN PATENTS OR APPLlCATlONS North Canada 46-3050] 9/1971 260/524 R [73] Assignee: Gulf Research & Development Company, Pittsburgh, Pa. Primary ExaminerAnton H. Sutto Filed. y 11 1972 Assistant ExaminerRichard D. Kelly [21] Appl. No.: 270,684 [57] ABSTRACT Related US Application Dam A process for selectively oxidizing the methyl substitur ent on a benzenoid structure carrying a nuclear [63] fgy sggfsgxz of methyl substituent and a higher alkyl substituent which involves heating the benzenoid structure with [52] Us. CL 260/524 260/515 260/599 oxygen in the presence of cobaltic ions dissolved in a lower carboxylic acid solvent. [5 l] Int. Cl. C074: 63/04 14 Claims, No Drawings PROCESS FOR OXIDIZING A BENZENOID STRUCTURE CARRYING A METHYL SUBSTITUENT This application is a continuation-in-part application of our application Ser. No. 32,335 filed Apr. 27, I970 entitled PROCESS FOR OXlDlZlNG A BENZENOID STRUCTURE CARRYING A METHYL SUBSTITU- ENT, now abandoned.

This invention relates to a process for selectively converting the methyl substituent on a benzenoid structure carrying a nuclear methyl substituent and a higher alkyl substituent to a carboxylic acid group.

It is well known in the art that there is no practical method of oxidation in the liquid phase that will result in the selective oxidation of a methyl group occurring as a substituent on a benzenoid ring in the presence of a side chain containing a plurality of carbon atoms. This is the conclusion of Palmer et al. in US. Pat. No. 2,302,462. Palmer et al. attempted to solve this problem by subjecting para-cymene to oxidation with molecular oxygen in the liquid phase, but in the absence of solvents, in the presence of selected oxidation catalysts. Although as catalysts manganese, cobalt, lead, iron, nickel, copper, vanadium, chromium and mercury were mentioned as being suitable in the process, in the sole example Palmer et al. used a mixture of manganese acetate and lead acetate as catalyst in the oxidation of para-cymene and obtained an oxidation mixture containing about 20 percent of its weight as para-cumic acid, the balance being some other oxidation products of cymene, such as dimethyltolyl carbinol and paramethylacetophenone, with yield of para-cumic acid, on the basis of cymene consumed in the oxidation process, amounting to about 40 percent. It is interesting to note that Palmer et al. in their US. Pat. No. 2,302,466 employ the identical run used in their US. Pat. No. 2,302,462, referred to above, to support equally well claims to the production of a mixture of alcohols and kctones.

Palmer et al.s results are similar to those obtained in Swedish Pat. No. 126,464, wherein oxidation of paracymene in the absence ofa solvent and catalyzed either by cobalt or manganese catalyst was reported to give up to 35 percent yield of paracumic acid and up to 60 percent of alcohol.

Similar results were also obtained by V. B. Falkovskii and L. A. Golubko, Neftekhimiya, 8 (3) 392 1968) who oxidized para-cymene in acetic acid in the presence of cobalt carbonate and sodium bromide at a temperature of l C. The weight of paracumic acid in the product was between 25 and 35 percent of the total, the remainder consisting essentially of paramethylacetophenone.

A study of the literature shows that competition between the isopropyl group and the methyl group in the oxidation of paracymene is greatly in favor of the isopropyl group (3.5: l as shown by G. Serif, C. Hunt and A. Bourns in Can. J. Clzenr, 3|, l229(l953); 3.221, as shown by G. A. Russell in J. Amer. Chem. 500., 78, 1047 (1956); and 4.2:], as shown by E. C. Kooyman, Disc. Faraday $00, No. I0, 163 (I95 I The benzenoid structure that is subjected to oxidation herein can be defined as one having an aromatic nucleus carrying as nuclear substituents thereon a methyl group and a higher alkyl group having from two to ten carbon atoms, preferably from two to 4 carbon atoms. The higher alkyl substituent is one wherein the carbon attached to the benzenoid structure carries sec ondary or tertiary hydrogens. Examples of such compounds are ortho-ethyltoluene, ortho-propyltoluene, ortho-isopropyltoluene, ortho-butyltoluene, orthoisobutyltoluene, ortho-sec-butyltoluene, 2-( lmethyloctyl )toluene, 2-( 2,2-dimethylheptyl )toluene, 2-( l-methylnonyl)toluene, 2-( 2,2-dimethyloctyl )toluene, meta-ethyltoluene, metapropyltoluene, metaisopropyltoluene, meta-butyltoluene, metaisobutyltoluene, meta-sec-butyltoluene, 3-( lmethyloctyl)toluene, 3-(2,2-dimethylheptyl)toluene, 3-( l-methylnonyl)toluene, 3-(2,2-dimethyloctyl )toluene, para-ethyltoluene, para-propyltoluene, paraisopropyltoluene(para-cymene), para-butyltoluene, paraisobutyltoluene, para-sec-butyltoluene, 4-( lmethyloctyl)toluene, 4-(2,2-dimethylheptyl)toluene, 4-( l-methylnonyl )toluene, 4-( 2,2-dimethyloctyl )toluene, etc. Of these we prefer to employ para-cymene as charge.

Since it would normally be expected that oxidation of the benzenoid compounds defined above would result in the conversion of both the methyl substituent and the higher alkyl substituents to carboxylic acid groups, it is critical herein that the conditions of reaction be strictly enforced in order to obtain high yields of desired product. The oxidation procedure involves heating the benzenoid compound in contact with a gas containing molecular oxygen in the presence of cobaltic ions dissolved in a lower carboxylic acid solvent.

The amount of oxygen used is at least that amount stoichiometrically required to convert the methyl substituent on the benzenoid compound to the carboxylic acid group. Complete utilization of oxygen may not occur in all cases, but it is possible to use amounts in excess of those stoichiometrically required, for example, from about 1.5 to about 20 molar excess.

Colbat can be used in the form of any of its compounds, preferably as a salt, soluble in the reaction mixture. Thus, the cobalt compound can be in the form of an inorganic compound or as an organic compound, for example, as a cobaltous or cobaltic chloride, sulfate, nitrate, acetate, propionate, butyrate, isovalerate, benzoate, toluate, naphthenate, salicylate, acetyl acetonate, etc. Of these we prefer to employ cobaltous or cobaltic acetate. The amount of cobalt compound, as cobalt, employed must be at least about 0.5 percent by weight, based on the reaction medium or solvent, defined hereinafter, preferably from about I to about 5 percent by weight or even higher.

The reaction medium or solvent is a lower carboxylic acid having from 2 to l6 carbon atoms, preferably 2 to 6 carbon atoms, such as acetic acid, propionic acid, nbutyric acid, n-valeric acid, n-caproic acid, n-heptanoic acid, n-caprylic acid, n-nonanoic acid, n-decanoic acid, n-undecanoic acid, n-dodecanoic acid, mtridecanoic acid, n-tetradecanoic acid, n-pentadecanoic acid, nhexadecanoic acid, etc. The amount of reaction medium used is at least that amount sufficient to render the reaction mixture into a substantially homogeneous liquid phase. Thus, the weight ratio of reaction medium to benzenoid compound can be from about lOO:l to about l:l, preferably from about 15:1 to about 4: I.

As to reaction conditions, the temperature can be from about 50 to about 200 C., preferably from about to about C. Pressure does not affect the course of the reaction and the only consideration thereof resides in employing sufficient pressure to maintain the desired liquid phase. A pressure of about 50 to about 1000 pounds per square inch gauge, preferably about lOO to about 400 pounds per square inch gauge, is sufficient. Reaction time is dependent upon the amount of conversion desired and generally the reaction is then terminated. Thus, a reaction time of about one minute to about 20 hours, preferably about l minutes to about three hours, can be used.

The process described and claimed herein will function only when the cobalt is in the form of a cobaltic ion. In the event cobaltous ions are in the reaction solu tion, it is necessary to convert the same to their cobaltic form. This can be done, for example, by having in the reaction mixture a promoter, such as a methylenic ketone. Examples of the same that can be used are methyl ethyl ketone, methyl propyl ketone, diethyl ketone, cyclopentanone, cyclohexanone, cycloheptanone, cyclooctanone, l-tetralone, 2-tetralone, etc. Of these we prefer methyl ethyl ketone, which can be added as such or can be made in situ from butane as the reaction proceeds. When a promoter is used it can be present in an amount of about 0.l to about I00 percent by weight, preferably from about one to about five percent by weight, based upon the reaction medium.

The reaction of this invention can be carried out in any suitable manner, batch or continuous, as long as intimate contact is maintained among the various components of the reaction system. Thus, in a batch system, the reaction medium, for example, acetic acid, paracymene and cobaltic acetate, are placed in a closed reactor and the same is pressured to reaction pressure with oxygen. The mixture is then raised to reaction temperature while stirring. Additional oxygen is introduced into the reaction system to compensate for the oxygen taken up by the reaction. Reaction is discontinued at any time but preferably when further oxygen absorption ceases. In a continuous reaction, for example, the acetic acid, para-cymene, cobaltic acetate and oxygen are passed upwardly through a reactor containing a sparger or an inert packing, such as Raschig rings or Berl saddles, maintained at suitable temperature and pressure to obtain the desired conversion. The reaction product is removed continuously overhead.

Recovery of desired product from the reaction mixture can be effected in any suitable manner. Thus, the total product can be cooled and then diluted with water to precipitate the desired oxidized benzenoid compound, which can then be recovered by simple filtration, after which precipitate can be washed with water and then dried. In a preferred embodiment the filtrate can be distilled to remove excess water therefrom and the remainder, constituting solvent and cobalt compound, can be reused in further oxidation.

The process can further be illustrated by the following:

EXAMPLE I Into a one-liter, 3l6-stainless steel, magneticallystirred autoclave there was placed 53 grams of paracymene, 20 grams of methyl ethyl ketone, 400 grams of acetic acid, 20 grams of cobaltous acetate tetrahydrate (l.2 percent by weight, as cobalt, based on the acetic acid) and 70 grams of normal butane (as the cooxidant to ensure substantially complete conversion While the mixture was vigorously stirred, the unit was pressured with oxygen to about 150 to 200 pounds per square inch gauge and heat was then applied thereto. When the temperature of the reaction mixture reached 105 C., more oxygen was added to the autoclave to raise the pressure to 320 pounds per square inch gauge. This pressure was maintained throughout the reaction period. After an induction period of 60 minutes the reaction began as evidenced by continuous oxygen absorption. After a reaction time of 1.5 hours at l05 C. the reaction was terminated, the autoclave was cooled to room temperature and depressured through a series of cold traps to remove unreacted normal butane. The contents of the autoclave were drained and most of the acetic acid solvent was removed under reduced pressure. About 500 cubic centimeters of ice water was added to the residue to cause a white solid acid to precipitate out of solution. The acid product was filtered, washed with water several times and dried in an oven to give 64.9 grams of product. The latter was analyzed by infrared and nuclear magnetic resonance spectroscopy, gas chromatography, melting point and neutral equivalent and was found to be percent by weight of the desired para isopropylbenzoic acid (cumic acid) and ten percent para-acetobenzoic acid. Conversion of the paracymene was complete. The selectivity to cumic acid was 90 percent.

EXAMPLE [I The run of Example I was repeated except that the cobaltous acetate was reduced to l0 grams (0.6 percent by weight based on the acetic acid) and normal butane was reduced to 65 grams. Induction period increased to 2l0 minutes. Analysis showed 18 percent conversion of para-cymene with the production of nine grams of para-isopropylbenzoic acid and L2 grams of para-methylacetophenone. The selectivity to cumic acid was 88 percent.

EXAMPLE ll] Again the run of Example I was repeated, this time with cobaltous acetate being reduced to give grams (0.3 percent by weight based on acetic acid) and normal butane was present in an amount of 68 grams. Para-cymene was increased to 60 grams. The induction period was 650 minutes. Conversion of para-cymene was less than two percent, with only a trace of paraisopropylbenzoic acid being found. This shows that the amount of cobalt used herein is critical and must be in excess of 0.3 percent by weight.

EXAMPLE IV The run of Example I was repeated, except that 20 grams of cyclohexanone was used in place of methyl ethyl ketone and only 52 grams of normal butane was present initially. The temperature was increased to 127 C. and the reaction time reduced to 0.25 hour. Induction time was negligible. Conversion of paracymene was percent and resulted in the production of 55 grams of para-isopropylbenzoic acid (86 percent yield), two grams of para-acetobenzoic acid (3.8 percent yield) and 5.4 grams of para-methylacetophenone (l0.2 percent yield).

EXAMPLE V The procedure of Example I was used with the following 20 grams of cobaltous acetate tetrahydrate l .2 percent by weight based on the acetic acid), 20 grams of methyl ethyl ketone, 400 grams of acetic acid, 60

grams of butane and grams of paraethyltoluene. The temperature was 105 C., the pressure 300 pounds per square inch gauge and the reaction time 0.67 hour. The induction period was 40 minutes. Analysis showed 95 percent conversion to the following: 0.7 gram (two percent yield) to para-acetobenzoic acid, l6 grams (68 percent yield) to para-ethylbenzoic acid, 1.3 grams (5.5 percent yield) to para-toluic acid and 5.5 grams percent yield) to para-methylacetophenone.

EXAMPLE VI This time Example 1 was repeated with the following: 20 grams of cobaltous acetate tetrahydrate (l.2 percent by weight based on the acetic acid), 20 grams of methyl ethyl ketone, 400 grams of acetic acid and I00 grams of secondary butyltoluene. The latter was 36.5 percent para, 59.0 percent meta and 4.5 percent ortho. The temperature was maintained at 105 C the pressure 320 pounds per square inch gauge and the reaction time 1.5 hours. The induction period was minutes. Conversion of secondary butyltoluene was 100 percent and resulted in 66 grams (89 percent yield) of secondary butylbenzoic acid, of which 38.3 percent was para, 59.5 percent was meta and 2.2 percent was ortho.

A study of the above clearly illustrates the advantage of operating in accordance with our procedure. In Example I there was obtained complete conversion with substantially all of the product to the desired compound. Reduction of the amount of cobalt compound used in Example [I] proved detrimental. Example IV was employed merely to show that other methylenic ketones can be used in place of methyl ethyl ketone. Examples V and Vl exemplify the present procedure with additional charge stocks.

That the selective oxidation sought herein is specific to the use of cobalt and a lower carboxylic acid solvent is apparent from the following:

EXAMPLE Vll This run was carried out similarly to the sole operating example in US. Pat. No. 2,302,462 to Palmer et al., referred to above. A mixture consisting of two parts by weight of manganese acetate tetrahydrate and one part of lead acetate tetrahydrate was ground into a powder and then dehydrated in a vacuum oven at l30 C. When dehydration was complete, the solid mass was cooled, ground into a fine powder and stored in a vacuum dessicator. A mixture consisting of 100 cc of paracymene and 100 cc of acetic acid containing four grams of the lead-manganese catalyst so prepared was charged into a 500 cc glass reactor equipped with a condensor, a thermometer well and a gas dispersion tube connected to an air supply. The reaction mixture was heated for two hours at 48 C., after which time the temperature was reduced to 30 C. and the reaction continued for 45 hours. At the end of the run, the crude product was poured into water and the organic layer extracted with ethyl ether. The ethereal layer was washed with water, dried with calcium chloride and the solvents removed on a rotary evaporator. Analysis of the residue by infrared spectroscopy showed that essentially no reaction occurred. Extraction of product with aqueous sodium hydroxide, followed by acidification with hydrochloric acid, gave no acids of paracymene.

EXAMPLE VIII boxylic acid to the reaction mixture similar to those of Examples VII and VIII is of little avail is shown by Example 1X.

EXAMPLE IX After an autoclave was charged with 20 grams of manganous acetate tetrahydrate, 400 grams of acetic acid, 20 grams of methyl ethyl ketone, 66 grams of para-cymene and 54 grams of normal butane, it was brought up to an operating temperature of l05 C. and an oxygen pressure of 300 pounds per square inch gauge. After an induction period of about one hour, the reaction was continued for 2 hours. The product mixture was worked up and analyzed as in Example I hereinabove. Analysis showed that about 29 percent of the para cymene was converted, with the product consisting of 13.3 grams (68 percent selectivity) of para-toluic acid, 3.l grams l6 percent selectivity) of paramethylacetophenone and 3.7 grams l 6 percent selectivity) of para-isopropylbenzoic acid.

That the use of cobalt herein without the presence of a lower carboxylic acid is of little avail is seen from Example X.

EXAMPLE X After an autoclave was charged with 20 grams of cobaltous acetate tetrahydrate, 70 grams of para-cymene, 20 grams of methyl ethyl ketone and grams of normal butane, it was brought up to an operating temperature of C. and an oxygen pressure of 300 pounds per square inch gauge. After an apparent induction period of one hour, the reaction was continued for l .5 hours. The autoclave was cooled, depressured and the product removed. Extraction of a small portion thereof with sodium hydroxide solution followed by acidification with hydrochloric acid gave no acidic products of para-cymene. Analysis of the crude mixture by infrared spectroscopy indicated para-methylacetophenone to be the only detectable product of the reaction.

From the above it can be seen, therefore, that it is critical that both cobaltic ions and a lower carboxylic acid be in the reaction mixture to obtain selective oxidation of the methyl substituent on a benzenoic structure also carrying a higher alkyl substituent.

That cobaltic ions in acetic acid are unique in their selectivity is most unexpected and could not have been predicted by reference to the prior art. The conclusion is reached that under conditions employed herein cobalt in acetic acid must function by a different mechanism that with manganese, manganese-lead mixtures, or cobalt alone in the absence of a carboxylic acid solvent.

Obviously, many modifications and variations of the invention, as hereinabove set forth, can be made without departing from the spirit and scope thereof, and

therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. A process for selectively oxidizing the methyl substituent on a benzenoid structure carrying a nuclear methyl substituent and a higher alkyl substituent, said higher substituent being one wherein the carbon attached to the benzenoid structure carries secondary or tertiary hydrogens which comprises heating said benzenoid structure with molecular oxygen in the presence of a catalyst consisting essentially of cobaltic ions in a lower carboxylic acid at a temperature of about 50 to about 200 C., said cobaltic ions being present in an amount of about one to about percent by weight based on said lower carboxylic acid.

2. The process of claim I wherein the reaction temperature is from about 85 to about l35 C.

3. The process of claim I wherein the lower carboxylic acid is acetic acid.

4. The process of claim 1 wherein a methylenic ketone is also present in the reaction mixture.

5. The process of claim I wherein methyl ethyl ketone is also present in the reaction mixture.

6. The process of claim 1 wherein cyclohexanone is also present in the reaction mixture.

7. The process of claim 1 wherein said higher alkyl substituent is ethyl.

8. The process of claim 1 wherein said higher alkyl substituent is isopropyl.

9. The process of claim I wherein said higher alkyl substituent is secondary butyl.

10. The process of claim I wherein said benzenoid structure is para-cymene.

11. The process of claim 1 wherein said benzenoid structure is para-ethyltoluene.

12. The process of claim 1 wherein the lower carboxylic acid is acetic acid, methyl ethyl ketone is also present and said benzenoid structure is para-cymene.

13. The process of claim 1 wherein the lower carboxylic acid is acetic acid, methyl ethyl ketone is also present and said benzenoid structure is a secondary butyltoluene.

14. The process of claim 1 wherein the lower carboxylic acid is acetic acid, methyl ethyl ketone is also present and said benzenoid structure is para-ethyltoluene. l 

1. A PROCESS FOR SELECTIVELY OXIDIZING THE METHYL SUBSTITUENT ON A BENZENOID STRUCTURE CARRYING A NUCLEAR METHYL SUBSTIUENT AND A HIGHER ALKYL SUBSTIUENT, SAID HIGHER SUBSTITUENT BEING ONE WHEREIN THE CARBON ATTACHED TO THE BENZENOID STRUCTURE CARRIES SECONDARY OR TERITARY HYDROGENS WHICH COMPRISES HEATING BENZENOID STRUCTURE WITH MOLECULAR OXYGEN IN THE PRESENCE OF A CATALYST CONSISTING ESSENTIALLY OF COBALTIC IONS IN A LOWER CARBOXYLIC ACID AT A TEMPERATURE OF ABOUT 50* TO ABOUT 200*C., SAID COBALTIC IONS BEING PRESENT IN AN AMOUNT OF ABOUT ONE TO ABOUT 5 PERCENT BY WEIGHT BASED ON SAID LOWER CARBOXYLIC ACID.
 2. The process of claim 1 wherein the reaction temperature is from about 85* to about 135* C.
 3. The process of claim 1 wherein the lower carboxylic acid is acetic acid.
 4. The process of claim 1 wherein a methylenic ketone is also present in the reaction mixture.
 5. The process of claim 1 wherein methyl ethyl ketone is also present in the reaction mixture.
 6. The process of claim 1 wherein cyclohexanone is also present in the reaction mixture.
 7. The process of claim 1 wherein said higher alkyl substituent is ethyl.
 8. The process of claim 1 wherein said higher alkyl substituent is isopropyl.
 9. The process of claim 1 wherein said higher alkyl substituent is secondary butyl.
 10. The process of claim 1 wherein said benzenoid structure is para-cymene.
 11. The process of claim 1 wherein said benzenoid structure is para-ethyltoluene.
 12. The process of claim 1 wherein the lower carboxylic acid is acetic acid, methyl ethyl ketone is also present and said benzenoid structure is para-cymene.
 13. The process of claim 1 wherein the lower carboxylic acid is acetic acid, methyl ethyl ketone is also present and said benzenoid structure is a secondary butyltoluene.
 14. The process of claim 1 wherein the lower carboxylic acid is acetic acid, methyl ethyl ketone is also present and said benzenoid structure is para-ethyltoluene. 